Laser coding of materials is well-known—see for example, U.S. Pat. No. 5,783,793, U.S. Pat. No. 4,906,813 and also U.S. Pat. No. 5,340,628 which seeks to contain the particles produced by ablation. These methods present a variety of problems, including difficulties in maintenance, line down-time, taint, as well as the need for extraction.
WO 02/068205 teaches a method for marking an object using a laser. Suitable additives, including for instance a polyhydroxy compound and a dehydrating compound are provided in a coating on a solid substrate, which is then imaged. Exemplary substrates include foodstuffs. The polyhydroxy agent is typically a sugar, and a metal salt may be used to remove OH groups.
WO 2007/045912 describes methods of marking substrates such as paper, card or board. The substrate is coated with a solution of a soluble alkali or alkaline earth metal salt of a weak acid, and areas of the substrate are irradiated such that those areas change colour.
In view of the prior art there remains a desire to provide improved marking methods which can be used on a wide variety of substrates.